Oh well, if we're all having a go...

You need to monitor some script, and at certain points start other scripts, and monitor those too?

open HANDLE, "some_script |";
push @INPUT, *HANDLE;
while (<@INPUT>) {
  if ($some_condition) {
    open HANDLE, "some_other_script |";
    push @INPUT, *HANDLE;
  }
  do_something_with_input();
}
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Where <@INPUT> is pseudo-code for reading from one of the files in @INPUT, without blocking.

The perl cookbook suggests:

use IO::Select;

$select = IO::Select->new();
# repeat next line for all filehandles to poll
$select->add(*FILEHANDLE);
if (@ready = $select->can_read(0)) {
    # input waiting on the filehandles in @ready
}
[download]

to perform this check. Putting the two together is left as an excercise for the reader.